Fractionation control



Nov. 24, `1.964

Filed June 16, '1960 i2 :Sheets-(Sheet .l

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E Hc. l lsflEA-M 'l 5o 69 l y .l z Q KETTLERRODUCT :MAGNETIC-rAMP-LIFIER "Qi 43x :ffm l t 40 .MAGNETIC l y AMPLTFTER f l T 68 1;,2451g II OGARflTl-TMTC- J 4| 46 5 E NETWORK LOGARITHMIC 5/- 1: 47 5| 5NETWORK 44'? IT L 5 2 v 54 35 f 65 6o 66 62 MAGNETTC l Eggfgk'AMPLIFIER. 453:57' iff 'LOGARITHMIC 3' l T NETWORK BYH g qv n 55E 1 e3if Q a F/G. 2 ATTORNEYS Nov. l24, 1964 E. D. TouN 3,155,8{557FRACTIONATIQN CONT-ROL Filed June 16, 1960 32 'Sheets-:Sheet f2 |05 loo107 los lol T F/G. .f5

A TTORNEYS United States Patent Cilce 3,158,557 FRACTEONATIUN CQNTROErnest D. Tulln, Bartlesville, Ghia., assigner to Phillips PetroleumCompany, a corporation of Delaware Filed .lune i6, 1960, Ser. No. 36,6024 Claims. (Cl. 2in-160) This invention relates to the computation ofinternal reilux in a fractionation column. In another aspect it relatesto control systems for fractionation columns which are based oncomputations of internal reilux.

In recent years an increasing use has been made of fan coolers forcondensing overhead vapors from fractionation columns. However, thistype of cooler has resulted in a rather serious operating problembecause it is dillicult to control the exact amount of cooling provided.Such schemes as fan speed control, variable pitch fan blade control andhot vapor by-pass control have been employed in an attempt to solve thisproblem, but have not been entirely satisfactory. Sudden atmospherictemperature changes, such as occur during a rainstorm, for example,result in a lowering of the reflux temperature. This causes an increasein the flow of liquid leaving the top tray because more of the vaporwhich enters this tray is condensed. This results in an increase inoverhead product purity at the expense of a decreased overhead productrate.

In accordance with the present invention there is provided a novelsystem for computing the amount of internal relux in a fractionationcolumn. Internal reilux constitutes the external reflux returned to thecolumn plus the vapor which is condensed near the top of thefractionation column by the sub-cooled external reflux. This computationis made from a measurement of the rate of ilow of the external refluxand a measurement of the ternperature differential between the externalreflux returned to the column and a region near the top of the column.Electrical signals are established which are representative of these twomeasurements. The electrical signals are combined to provide ameasurement of the internal rellux in the column. A signalrepresentative of this internal rellux can be employed to control theoperation of the column to maintain desired steady state operatingconditions. This control can be made automatically or by an operatorfrom the information provided.

Accordingly, it is an object of this invention to provide a system forcomputing the internal reflux in a fractionation column.

Another object is to provide control systems for fractionation columnswhich utilize computations of the internal reflux in the column.

Other objects, advantages and features of the invention should becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawing in which:

FIGURE l is a .schematic representation of a fractionation column havingthe computer and control system of this invention associated therewith.

FIGURE 2 is a schematic circuit drawing of the computer employed in thecontrol system of FIGURE l.

FIGURE 3 is aschematic circuit drawing of a positive logarithmic networkemployed in the computer of FIG- URE 2.

FIGURE 4 is a schematic circuit drawing of a negative logarithmicnetwork employed in the computer of FIG- URE 2. f

. FIGURE 5 is a schematic circuit drawing of the magnetic amplier whichis employed in the computer of FIG- URE 2.

Referring now to the drawing in detail and to FIGURE i 1 in particular,there is shown a conventional fractionation column 10 which contains anumber of vaporliquid contacting trays. A iluid mixture to be separatedis intro- 3,158,557 Patented Nov. 2.4, 1964,

duced into column l0 through a conduit 11 at a predetermined rate whichis maintained by a ilow controller 12 that adjusts a valve 13. Steam, orother heating medium, is circulated through the lower region of column10 through a conduit 14 at a predetermined rate which is maintained by allow controller 15 that adjusts a valve 16.

Vapors are withdrawn from the top of column 10 through a conduit 18which has a condenser 19 therein. Conduit 18 communicates at its secondend with an accumulator 2li. A portion of the condensate in accumulator20 is returned to column 10 as external rellux through a conduit 2l. Theremainder of the condensate is removed as overhead product through aconduit 22 which has a control valve 23 therein. Valve 23 is adjusted bya liquid level controller 24 to maintain a predetermined level inaccumulator 2G. A kettle product stream is withdrawn from the bottom ofcolumn l@ through a conduit 26 which has a control valve 27 therein.Valve 27 is adjusted by a liquid level controller 28 to maintain apredetermined liquid level in the bottom of column 1G.

In order to explain-the operationof the internal reilux computer andcontrol system of this invention, an equation which is representative ofthe internal reflux in a fractionation column will be derived.

The material balance at the top tray of the fractionator can beexpressed:

Where Re=mass flow of liquid entering top tray (external reflux) Vi=massllow of vapor entering top tray R1=mass ilow of liquid leaving top tray(internal reflux) Vo=mass llow of vapor leaving top tray.

The heat balance at the top tray can be expressed:

Rehe-t V1H=Rihi+VoH (2) where he--enthalpy of external relluxh1=enthalpy of internal reflux H :enthalpy of vapor streams (assumed tobe equal).

The enthalpy of the vapor streams entering and leaving the top tray canbe expressed:

where Cp=specic heat of the external reflux stream AT=the difference intemperature between the top tray and external rellux.

Equation 3 can be substituted into Equation 2 to eliminate H andrewritten:

Equation 4 can be substituted into Equation 5 Vto eliminate he andrewritten: f

(han (perch/tirava) naar 6) From Equation 1 it is known:

Vi- V0=R1Re (7 Equation 7 can be substituted into Equation 6 and reducedto obtain:

The computer of this invention solves Equation 8. The term Re isobtained from a measurement whichis representative of the rate of flowthrough. conduit 21.

This is accomplished, for example, by the use of a differential pressuretransmitter 30 which establishes. an output electrical signalrepresentative of the differential pressure across an orifice in conduit21. It is assumed that the temperature of the vapor leaving the top-tray of Column is equal to the temperature of the liquid on the toptray. The term AT is then measured by comparing the vapor temperaturewith the external reflux temperature. Thermocouples 31 and `32 areconnected in opposing relationship to the input of a differentialtemperature transducer 33. This transducer can be a suitable amplifierto provide an output current of sufficient magnitude to be employed inmaking the internal reflux computation. The output signals fromtransducers 30 and 33 are applied to the inputs of a computer 34. n

Computer 34 operates in the manner described hereinafter to establish anoutput signal representative of the internal refiux in the fractionationsystem. The output signal from computer 34 is employed to adjust the setpoint of a controller 35. Controller 35, in turn, adjusts a controlvalve 36 in reflux conduit 211. The control system operates to maintainthe computed internal reflux constant at a preselected value. If thecomputed internal reflux should increase, valve 36 is adjusted to reducethe ow through conduit 21. If the computed internal reflux shoulddecrease, valve 36 is adjusted Ito increase the flow through conduit 21.

Computer 34 is illustrated schematically in P IGURE 2. The outputelectrical signal from differential pressure transducer 30 is applied torespective input terminals 40 and 41 of the computer. As is Well known,the differential pressure measurement across an orifice is proportionalto the square of the flow through the conduit. Terminal 40 is connectedto the contactor of a potentiometer 42. Terminal 41 is connected to thegrounded lfirst end terminal of the potentiometer. The contactor ofpotentiometer 42 is set so that the input signal is multiplied by aconstant K2 which is representative of the orifice flow constant. Thisis accomplished by adjusting the flow of input current through the twoarms of the potentiometer. The current flow through the upper arm isthus representative of the term KZAP. This signal is applied to theinput of a magnetic amplifier 43. A reference bias signal is applied tothe input of amplifier 43 from a constant current source formed by abattery 44 and an adjustable resistor d5. Amplifier 43 is provided witha feedback resistor 4e, and a resistor d'7 is connected between thisfeedback resistor and ground. The output of signal of amplifier 43 isapplied through a resistor 49 to the input of a second magneticampli-fier 50. A first logarithmic network y51 and a resistor 52 areconnected between the output of amplifier 43 and ground. Resistors 49and 52 are of equal value so that one half of the output signal fromamplifier 43 is shunted to ground. Therefore, the signal applied toamplifier 5u is representative of the logarithm of the quantity \/K2AP.

The output electrical signal of transducer 33 is applied betweenterminals 52 and 53 of the computer of FIG- URE 2. Terminal 52 isconnected through a resistor 54 to the input of a third magneticamplifier 55. Resistor 54 is selected so that this input signal isattenuated so as to be multiplied by the constant tive of the term (l ATof Equation 8. The output signal of amplifier 55 is -applied through aresistor 60 to the input Of magnetic amtity Q2 (1+ AT being applied toamplifier 50.

A bias signal is applied to amplifier 50 from a constant current sourceformed by a battery 65 and a variable resistor 66. Amplifier 50 isprovided with a feedback resistor 67, and a logarithmic network isconnected between this feedback network and ground. Amplifier 50 servesas a summing amplifier to sum the logarithmic output signals fromamplifiers 43 and 55. Logarithmic network 68 in the feedback path ofamplifier 50 results in the output signal from amplifier 50 beingrepresentative of the antilogarithrn of the summed logarithms. Thisoutput sig nal, which is applied to terminals 69 and 70, is thusrepresentative of the computed internal reflux.

The two logarithmic networks 51 and 61 are positive logarithmic networkswhich can be of the form illustrated in FIGURE 3. This network isprovided with an input terminal which is connected directly to an outputterminal 76. Terminals 75 and 7 6 are connected to the Ifirst terminalsof respective rectitiers 77, 78, 79, and 81. The positive terminal of avoltage source 82 is connected to ground through series connectedresistors 83, 84, 85, 86 and 87. The negative terminal of voltage source8-2 is connected directly to ground. The second terminal of rectifier 77is connected through a resistor 89 to the positive terminal of voltagesource 82. A resistor 90 is connected between the second terminal ofrectifier 78 and the junction between resistors 83 and 84. A resistor 91is connected between the second terminal of rectifier 79 and thejunction between resistors 84 and 85. A resistor 92 is connected betweenthe second terminal of rectifier 89 and the junction between resistors85 and 86. A resistor 93 is connected between the second terminal ofrectifier S1 and the junction between resistors 86 and 87.

The circuit illustrated in FIGURE 3 is a conventional logarithmicnetwork. The resistors 83 ot S7 serve to bias the rectifiers so thatthey normally do not conduct. Thus, for low values of input signals, allthe input current flows from terminal 76 through the external loadcircuit. As the input current -is increased, rectifier 81 begins toconduct so that some of the current is shunted to ground throughresistor 93. As Ithe input current increases still further, additionalrectifiers begin to conduct in sequence so that more of the current isshunted to ground. A logarithmic transfer characteristic can .thus beapproximated by a series of straight line segments provided by thesequential conduction of the rectifiers. Almost any desired degree ofaccuracy can be obtained by increasing the number of rectifiers in thecircuit.

Logarithrnic network 68 of FIGURE 2 is illustrated schematically inFIGURE 4. This network is similar in many respects to the network ofFIGURE 3 and corresponding elements are designated by like primedreference numerals. It should be observed, however, that the rectifiersare connected in the network in the opposite direction and the polarityof voltage source S2 is reversed from that shown in FIGURE 3.

A suitable magneticV amplifier for use in the computer of FIGURE 2 isillustrated schematically in FIGURE 5. This amplifier includes six coils100, 101, 102, 103,112, and 115. Coil is the control winding of thecircuit and lthe input current to the amplifier flows through this coilfrom input terminal to ground. A variable resistor 106 is connectedbetween a potential terminal 107 and the first terminal of bias coil101. The second terminal of coil 101 is connected to ground. Apredetermined current iiows through coil 101 to bias the amplifier. Coil102 provides voltage compensation in the amplifier. This coil isenergized from a source of alternating current 108 which is connectedacross the primary winding 109 of a transformer 110. One end terminal ofthe secondary winding 111 is connected to the external tap of winding111 through coil 112, a rectifier 113 and a capacitor 114. The secondend terminal of transformer winding 111 is connected to the external tapof this winding through coil 115, a rectifier 116 and capacitor 114. Thefirst end terminal of `transformer winding 111 is connected to thesecond end terminal of this winding through a rectifier 117, a resistor118 and coil 102. A capacitor 119 is connected across resistor 118 andcoil 102. The amplifier is provided with a feedback coil 103 which hasone terminal connected through a resistor 120 to the junction betweenrectifier 113 and capacitor 114. This junction is also connected to anoutput terminal 122. The second terminal of coil 103 is connected Itothe grounded center tap of transformer winding 111.

Coils 112 and 115 are mounted on separate cores 112 and 115',respectively, in opposing relationship. First halves of each of coils100, 101, 102 and 103 are mounted on core 112 in series-aidingrelationship; and second halves of each of coils 100, 101, 102 and 103are mounted on core 115 in series-aiding relationship.

Magnetic amplifiers are employed in the computer of this invention inorder to provide a simplified circuit which is highly reliable. However,it should be evident that conventional voltage amplifiers can beemployed in this computer if desired.

In view of the foregoing description it should be evident that there isprovided in accordance with this invention an improved refiux computerwhich employs electrical computing elements. While the invention hasbeen described in conjunction with present preferred embodiments, itshould be evident that it is not limited thereto.

What is claimed is:

1. In a fractionation system wherein a feed mixture of two or morecomponents is directed to a fractionation column, a vapor stream isremoved from the top of said column, said vapor stream is cooled tocondense at least a part of same, and at least a pant of the resultingcondensate is returned to said column as external reflux; a system tocompute the internal reux in said column comprising first means toestablish a first electrical signal representative of the rate of fiowof external reflux to said column, second means to establish a secondelectrical signal representative of the temperature difference betweensaid vapor stream and said external refiux, third means responsive tosaid rst means to establish a third electrical signal representative ofthe logarithm of said first signal, fourth means responsive to saidsecond means to increase said second signal by a preselected amount andthen to establish a fourth electrical signal representative of thelogarithm of the increased second signal, fifth means responsive to saidfthird and fourth means to establish a fifth electrical signalrepresentative of the sum of said third and fourth signals, and sixthmeans responsive :to :said fifth means to establish a sixth signalrepresentative of the antilogarithm of said fifth signal, said sixthsignal being representative of said internal reux, said first and thirdmeans comprising an orifice positioned in the external refiux flow,means to establish a seventh signal representative of the pressuredifferential across said orifice, means to multiply said seventh signalby a constant to establish an eighth signal, means to establish a ninthsignal representative of the logarithm of said eighth signal, and meansto divide said ninth signal into two equal signals, one of which is saidthird signal.

2. In a fractionation system wherein a feed mixture of two or morecomponents is directed to a fractionation column, a vapor stream isremoved from the top of said column, said vapor stream is cooled tocondense at least a part of same, and at least a part of the resultingcondensate is returned to said column as external reflux; a system tocompute the internal refiux in said column comprising first means toestablish a rst electrical signal representative of the rate of flow ofexternal reflux to said column, second means to establish a secondelectrical signal representative of the temperature difference betweensaid vapor stream and said external reflux, a potentiometer, first andsecond amplifiers,`means applying said first signal across saidpotentiometer, means connecting the contacter and one end terminal ofsaid potentiometer to the rinput of said first amplifier, means applyingsaid second signal tothe input of said second amplifier, a summingamplifier having a logarithmic network in the feedback network thereofso that the output signal of said summing amplifier is the antilogarithmof the input signal thereto, means including a second logarithmicnetwork connecting the output of said rst amplifier to the first inputof said summing amplifier, and means including a third logarithmicnetwork connecting the output of said second amplifier to the secondinput of said summing amplifier.

3. 'Ihe system of claim 2 wherein said first and second amplifiers andsaid summing amplifier are magnetic amplifiers.

4. The system of claim 2, further comprising means responsive to theoutput signal of said summing amplifier to control the rate of Vfiow ofsaid external reflux to increase the fiow of said external reflux whensaid sixth signal decreases and to decrease the flow of said externalreflux when said sixth signal increases.

References Cited in the file of this patent Lupfer et al.: ComputerControl of Distillalton Refiux, ISA JournaL'vol. 6, #6, pages 34-39,June 1959.

1. IN A FRACTIONATION SYSTEM WHEREIN A FEED MIXTURE OF TWO OR MORECOMPONENTS IS DIRECTED TO A FRACTIONATION COLUMN, A VAPOR STREAM ISREMOVED FROM THE TOP OF SAID COLUMN, SAID VAPOR STREAM IS COOLED TOCONDENSE AT LEAST A PART OF SAME, AND AT LEAST A PART OF THE RESULTINGCONDENSATE IS RETURNED TO SAID COLUMN AS EXTERNAL REFLUX; A SYSTEM TOCOMPUTE THE INTERNAL REFLUX IN SAID COLUMN COMPRISING FIRST MEANS TOESTABLISH A FIRST ELECTRICAL SIGNAL REPRESENTATIVE OF THE RATE OF FLOWOF EXTERNAL REFLUX TO SAID COLUMN, SECOND MEANS TO ESTABLISH A SECONDELECTRICAL SIGNAL REPRESENTATIVE OF THE TEMPERATURE DIFFERNCE BETWEENSAID VAPOR STREAM AND SAID EXTERNAL REFLUX, THIRD MEANS RESPONSIVE TOSAID FIRST MEANS TO ESTABLISH A THIRD ELECTRICAL SIGNAL REPRESENTATIVEOF THE LOGARITHM OF SAID FIRST SIGNAL, FOURTH MEANS RESPONSIVE TO SAIDSECOND MEANS TO INCREASE SAID SECOND SIGNAL BY A PRESELECTED AMOUNT ANDTHEN TO ESTABLISH A FOURTH ELECTRICAL SIGNAL REPRESENTATIVE OF THELOGARITHM OF THE INCREASED SECOND SIGNAL, FIFTH MEANS RESPONSIVE TO SAIDTHIRD AND FOURTH MEANS TO ESTABLISH A FIFTH ELECTRICAL SIGNALREPRESENTATIVE OF THE SUM OF SAID THIRD AND FOURTH SIGNALS, AND SIXTHMEANS RESPONSIVE TO SAID FIFTH MEANS TO ESTABLISH A SIXTH SIGNAL